1. Field of the Invention
The present invention relates to a computer system comprising physical links, and more specifically, to a computer system in which physical links for SAS (Serial Attached SCSI)-compliant communications are provided in the backend, for example.
2. Description of the Related Art
A storage system that comprises a plurality of storage devices (disk drives such as hard disk drives and DVD (Digital Versatile Disk, for example) is known as one type of computer system. A storage system is capable of receiving an I/O command (an I/O command constituting a write command or read command) from an access source device (a host computer or other storage system, for example) and of transmitting data corresponding with the received I/O command to the access source device by writing the data to a storage device or reading the data from a storage device.
The disk device disclosed in the Japanese Patent Application Laid Open No. 2005-149173, for example, is known as a storage system of this kind. In this disk drive, a plurality of disk drives are connected to a Fiber Channel-Arbitrated Loop (FC-AL).
Further, SAS (Serial Attached SCSI) is known as a communication I/F. SAS is an I/F technology that allows disk drives of different protocol types (SCSI and ATA, for example) to be connected by the same physical wiring. As a computer system that performs SAS-compliant communications to be performed by the backend, a storage system that comprises a plurality of disk drives and a controller that controls access to each disk drive may be considered. In this case, providing the storage system with an SAS expander comprising a plurality of physical communication ports (‘physical phys’ hereinbelow) that is a switch for connecting disk drives for expansion may be considered. A plurality of SAS expanders can be provided and, when a controller is provided at the highest point upstream, a plurality of SAS expanders can be cascade-connected working downstream from the highest point upstream (connected in series or as a tree structure, for example). However, the controller need not be at the highest point upstream and an SAS initiator can be connected to any point of the SAS expander. A controller, an upstream SAS expander, downstream SAS expander, or disk drive can be connected to an optional physical phy of each SAS expander.
In this storage system, physical wiring (‘physical link’ hereinbelow) is run between the controller and an SAS expander, between SAS expanders, and between an SAS expander and disk drive in the backend. When a disk drive (SAS target) is accessed, the controller (SAS initiator) is able to access a disk drive via each physical link from the controller to the disk drive. More specifically, for example, when the disk drive connected to the first SAS expander directly below the controller is accessed, the controller is able to access the disk drive via a first physical link that links the controller and the first SAS expander and a secondary physical link that links the first SAS expander and the first SAS expander and the disk drive.
SAS includes a technology known as ‘wide link’ that makes it possible to collect a plurality of parallel physical links between single devices as one logical link. That is, a wide link can be formed by a plurality of parallel physical links that connect one device with another device such as between an SAS expander and another SAS expander and between an SAS initiator and an SAS expander. Further, a wide link can also be formed automatically between an SAS expander and another SAS expander or between an SAS initiator and an SAS expander without an SAS-initiator instruction.
Further, SAS includes a technology that makes it possible to form a plurality of virtual physical links (‘logical links’ hereinbelow) through time division of a single physical link. Therefore, a plurality of logical links can be constituted in individual physical links forming a wide link. For example, in a case where two physical links with a transfer speed of 6 Gbps are formed by a single wide link, because a 1.5 Gbps logical link with ¼ the transfer speed can be formed by means of a single physical link, a total of eight logical links can be formed in the wide link. In this case, the SAS initiator device is capable of sending and receiving frames simultaneously by establishing a connection with eight SAS target devices at the same time.
The controller is able to establish a connection at the same time with a plurality of devices via a plurality of physical links constituting the wide link. When the controller and SAS expander are connected by a wide link and a plurality of disk drives are connected to the SAS expander, the controller is able to simultaneously establish connections with a plurality of disk drives in a quantity equal to the number of physical links constituting the wide link. One controller is able to execute a frame transfer to a plurality of disk drives at the same time by means of a wide link.
A characteristic problem can arise with this storage system in each of the first and second cases below, for example.
The first case is a case where disk drives with different communication I/Fs are connected to the SAS expander. For example, this case is a case where a disk drive with an SAS I/F (‘SAS drive’ hereinbelow) and a disk drive with a SATA (Serial ATA) I/F (‘SATA drive’ hereinbelow) are connected to one or a plurality of SAS expanders, for example. This case can be implemented by using the same physical link to transfer an SSP (Serial SCSI Protocol) frame for accessing an SAS drive or SAS device and an STP (Serial ATA Tunneled Protocol) frame that tunnels the SATA protocol for accessing the SATA drive. Here, SATA and SAS have different transfer efficiencies (specifically, the transfer efficiency of SATA is inferior to the transfer efficiency of SAS and the time occupied by the physical link is longer). As a result, when there are a large number of SATA-frame I/O transfer requests, there is the possibility of an adverse effect on the throughput with respect to the SAS drive. A problem of this kind is not limited to these two types of I/F and can also exist between I/Fs of other types.
The second case is a case where two or more disk drives are one group (‘RAID group’ hereinbelow) that follows the rules of RAID (Redundant Array of Independent (or Inexpensive) Disks) and the RAID groups are connected to an SAS expander. Because a plurality of SAS expanders are cascade-connected, a wide link of a path from the controller to a certain RAID group and a wide link of a path from the controller to another RAID group are common on the upstream side within the backend. As a result, when access to a specified RAID group increases, there is the possibility of an adverse effect on the throughput of the other RAID groups.
The above problems are not limited to storage systems and can also exist in other types of computer system. For example, in a case where a plurality of SAS target devices exist in a computer system, when a specified SAS target device is accessed, the possibility of the throughput of the computer system dropping may be considered.
Furthermore, the procedure for selecting a physical link for executing a frame transfer from a plurality of physical links that constitute a wide link based on the restrictions of protocols such as SAS cannot be controlled by an SAS initiator device.